Currently, color images are generated through a wide variety of different systems, such as for example photographically on suitable film or photosensitive paper, or electronically on video tape or other suitable media. When generated, images share a basic characteristic: they are recorded on a continuous tone (hereinafter referred to as "contone") basis. As such, recorded color information at any point in the image is represented by several continuous amplitude values, each of which is oftentimes discretized as eight-bit values ranging from "0" to "255". Very often, a user having an image captured on one medium, such as a photographic print or transparency, will desire to display and/or reproduce that image on other media, such as on a video monitor or on a printed page.
Color reproduction equipment, as it relates to printing images, takes advantage of the principle that the vast majority of colors can be separated into a specific combination of four primary subtractive colors (specifically cyan, yellow, magenta and black--C, Y, M and K) in which the amount of each primary color is set to a predetermined amount. In the case of printed reproductions of an image, use of primary color printing obviates the need to use a differently colored ink for each different color in the image. As such, each image is commonly converted into sets of three or four color separations, in which each separation is essentially a negative (or positive) transparency with an altered tone reproducing characteristic that carries the color information for only one of the primary colors. Separations are subsequently recorded on printing plates for use in a press.
By way of contrast, color reproduction on cathode ray tube displays takes advantage of the principle that the vast majority of colors can be represented by a combination of three primary additive colors (specifically red, green and blue--R, G and B) in which the intensity produced by each primary colored (R, G or B) phosphor is set to a predetermined amount.
Modern offset printing presses do not possess the capability of applying differential amounts of ink to any location in an image being printed. Rather, these presses are only designed to either apply or not apply a single amount of ink to any given location on a page. Therefore, an offset printing press is unable to directly print a contone separation. To successfully circumvent this problem, halftone separations are used instead. An image formed from any single color halftone separation encodes the density information inherent in a color image from amplitude modulated form into a spatial (area) modulated form, in terms of dot size, which is subsequently integrated by the human eye into a desired color. By smoothly changing halftone dot sizes (dot areas), smooth corresponding tone variations will be generated in the reproduced image. Given this, the art has taught for some time that a full color image can be formed by properly overlaying single color halftone reproductions for all of the primary subtractive colors, where each reproduction is formed from a corresponding halftone dot separation that contains dots of appropriate sizes. Clearly, as size and spacing of the dots decrease, an increasing amount of detail can be encoded in a halftone dot pattern and hence in the reproduced image. For that reason, in graphic arts applications, a halftone separation utilizes closely spaced dots to yield a relatively high resolution.
With this in mind, one might first think that printing a color image for graphic arts use should be a fairly simple process. Specifically, a color image could first be converted into corresponding continuous tone separations. Each of these contone separations could then be converted into a corresponding halftone separation. A printing plate could then be manufactured from each halftone separation and subsequently mounted to a printing press. Thereafter, paper or other similar media could be run through the press in such a fashion so as to produce properly registered superimposed halftone images for all the subtractive primary colors thereby generating a full color reproduction of the original image.
In practice, accurately printing a color image is oftentimes a very tedious, problematic and time consuming manual process that requires a substantial level of skill. First, the conventional manual photographic process of converting a contone separation into a halftone separation, this process commonly being referred to as "screening", is a time and resource consuming process in and of itself. Second, various phenomena, each of which disadvantageously degrades an image, often occur in a reproduced halftone color image. Moreover, the complete extent to which each of these phenomena is present in the reproduced image is often known only at a rather late point in the printing process thereby necessitating the use of tedious and time and resource consuming iterative experimentation to adequately eliminate these phenomena.
Traditionally, on-press proofing provided the first point at which a color judgment could be made regarding the quality of the reproduced image. For example, many color differences, such as incompatible and/or objectionable color renditions or Moire patterns, were usually first seen at this point in an imaging process. If such a difference were sufficiently objectionable to a color technician, then usually the entire imaging process would need to be modified and repeated. Doing so generally necessitated a total re-work of the separations, production of a new set of printing plates therefrom and generation of a new press proof, with this process being iteratively repeated as many times as necessary to properly remove or sufficiently attenuate the incompatible and/or objectionable color differences.
In an effort to reduce the time required and expense associated with conventional manual photographic based color reproduction processes and particularly the traditional on-press proofing techniques used therewith, the art has initially turned away from use of on-press proofing in high volume graphic art applications towards the use of intermediate off-press proofing technologies, such as electro-photographic techniques. In this regard, U.S. Pat. No. 4,708,459 (issued to C. Cowan et al on Nov. 24, 1987, assigned to the present assignee hereof and hereinafter referred to as the '459 Cowan et al patent) discloses an electro-photographic color proofing system with variable tone reproduction characteristics.
Now, for a variety of reasons, such as for example, recently increasing use of digital techniques in color electronic pre-press systems; and increasing flexibility, control and throughput over that provided by optical (including electro-photographic) proofing systems, the art appears to be turning towards the use of so-called direct digital color proofing (DDCP) systems. These particular systems directly generate a halftone color proof image from a set of digitized contone separations and particularly the digitized contone values therefor. Specifically, DDCP systems manipulate the separations in digital form to electronically generate appropriate halftone separations, including, inter alia, through use of electronic screening and tone reproduction compensation, and then directly write the proof image using an appropriate high resolution binary marking engine. Furthermore, inasmuch as these systems completely eliminate photographic film-based processes, these systems are expected to be very economical to operate.
By virtue of providing input-output mapping in a completely digital fashion, such DDCP systems should provide far better control over image subtleties and hence tone reproduction than that available through optical proofing systems known in the art.
In that regard, I have previously developed a technique for inclusion in, illustratively, a DDCP system that: (a) allows an operator to completely specify and readily change a desired tone reproduction curve shape that, within the physical limits of the system, is to be reproduced in the proof, and (b) causes the system produce a proof image that exhibits the desired dot gain curve shape. That technique is fully described in my co-pending U.S. patent application "A TECHNIQUE FOR USE IN CONJUNCTION WITH AN IMAGING SYSTEM FOR PROVIDING ACCURATE TONE REPRODUCTION IN AN OUTPUT IMAGE" filed Oct. 25, 1991, Ser. No. 07/782,940 (hereinafter referred to as the '940 Spence application) and which has been assigned to the present assignee hereof.
Very broadly speaking, this technique relies on intentionally varying the value of each incoming contone value by an amount consistent with both an actual tone reproduction characteristic of a DDCP imaging chain (i.e. a so-called "Process" dot gain) and a desired (so-called "Aim") dot gain to yield an output dot of an appropriate area that provides the desired density in the proof image. In this context, the DDCP imaging chain is illustratively formed of a raster image processor (RIP), which implements a screening process, and a marking engine connected thereto such as a sublimation dye transfer laser writer. To readily accomplish this variation, each incoming contone value is appropriately modified through illustratively a table look-up operation into a correspondingly modified value which, when subsequently rendered into a halftone pattern on the proof image by the marking engine, causes the proof to accurately exhibit the desired "Aim" tone reproduction curve. The look-up table contains values which represent the "Aim" tone reproduction curve modified by an inverse of the "Process" tone reproduction curve.
While the optical proofing system described in the '459 Cowan et al patent and the direct digital proofing system described in the '940 Spence application provide excellent quality proofs, these systems, like all imaging systems, can reproduce colors only within a certain color gamut. Generally speaking, the tone reproduction characteristics of one type of imaging system, or even one type of imaging medium, are not completely coincident with those of a different type of imaging system or medium. In this regard, through use of differing colorants (e.g. inks used in printing as compared to photographic dyes or colored phosphors on a video monitor) and other physical phenomena related to specific imaging processes, a given color shown on a color artwork, such as on a photograph, on any of a variety of other off-press proofs, on a press proof or on a press sheet printed on publication stock, will often appear differently in a halftone color proof formed either on electro-photographic film or on a dye transfer intermediate and subsequently transferred to paper. Furthermore, a halftone color proofing system, such as that described in the '459 Cowan et al patent or that described in the '940 Spence application, is generally incapable of producing the exact same color gamut and color response which are available through either the photograph, the other off-press proofs, the press proof or the press sheet. In this regard, the color gamut reproducible in a color halftone proof will generally not match that associated with a color artwork that appears on a photograph, an off-press proof, a press proof or on a press sheet. In addition and owing to physical differences among different imaging systems, the response of different types of imaging systems to an identical input color will likely be different, e.g. the same red color provided as input to two different imaging systems might likely produce two output colors with somewhat differing red hues.
In view of the inherent tone and color differences between, e.g., the press sheet and the proof thereof, the colors in the proof can not be identically matched to those that appear in the press sheet. Nevertheless, for a proofing system to fully serve its intended purpose, a proof image must be predictive of color rendition in images subsequently produced by another imaging system (hereinafter respectively referred to as a target images and target imaging systems). However, the tone and color reproduction characteristics of a proofing system rarely coincide with those of an associated target imaging system. Therefore, the tone and color reproduction characteristics of the proofing system must be calibrated, to the extent possible, to those of the target imaging system. Once calibrated, the proofing system should be able to accurately predict the performance of the target imaging system though, in most situations, the proofing system will generate a proof image with colors that do not exactly match those in a target image.
Unfortunately, calibrating a proofing system tends to consume an inordinate amount of time as well as require a very high level of skill. In this regard, a color technician is required to possess a substantial level of skill and expertise not only to judge color differences between a proof image and a target image therefor but also to fully appreciate performance inter-relationships between the colors that appear on the proof image and corresponding colors that will appear on the target image. Consequently, the technician not only must recognize a color difference and decide which specific colors to match but also, where the tone and color reproduction characteristics of the proofing system can be varied, determine the proper variations in these characteristics in order to achieve an acceptable match between the proof image and the target image and then set the proofing system accordingly.
In particular, to calibrate a proofing system to a target imaging system, a color technician usually visually examines both a proof image and an associated target image on a side-by-side basis and then, based upon his own subjective judgment as to what the visually important features of the target image are and how they should appear, selects which colors to match. Thereafter, given his knowledge of the proofing system and its color response, he will attempt to initially vary the C, Y, M and K colorant solid area densities and/or dot size (tone reproduction curve) settings to accurately depict one color(s), which, not surprisingly, will also affect other colors, possibly adversely. Based upon the effects that occur with respect to other colors in the proof image, the technician will iteratively vary solid area densities and/or dot size (tone reproduction curve) settings of the colorants, in seriatim, until an acceptable color match is achieved between the target image and the proof image for the selected colors.
However, a proofing system with variable tone and color reproduction characteristics often presents the technician with an enormous number of different possible combinations of the settings. For example, for the system described in the '459 Cowan et al patent, the solid area density and dot size can be set for each of the four process colors (C, Y, M and K) at any of 20 different density levels and at any of 15 different dot size settings. For the DDCP system described in the '940 Spence application, the number of solid area density settings is considerably larger, with, e.g., the number of dot size settings alone (comprising specification of several control points) numbering well into the thousands. In view of the resulting huge number of potential combinations of settings, an experienced color technician often needs to run and separately analyze quite a few successive proofs in order to select a suitable solid area density and halftone dot size setting (or an entire tone reproduction curve shape) for each different colorant in order to achieve an acceptable match between the proof image and a target image and thereby calibrate the proofing system to the target imaging system. Moreover, additional time is consumed whenever the technician is forced to resort to trial-and-error experimentation or, in a worst case scenario, guesswork: either merely as a result of iterating through a very large number of possible combinations to discern the performance inter-relationships of the proofing system and/or by incorrectly relying on intuition and initially iterating away from a proper operating condition. An example of the latter situation can occur where the technician, based upon his own intuition, views a proof image against a target image and decides that the yellow content in the proof image needs to be increased. While the technician may decide to initially increase the halftone dot size for the yellow colorant, the proper operating condition may instead involve reducing the halftone dot sizes for all the colorants but reducing the halftone dot size for yellow less than that for each of the other colorants.
Furthermore with certain images, the technician may simply have insufficient skill to quickly determine the proper operating conditions of the proofing system. As such, in certain situations, the technician, given his lack of knowledge or experience, may be unable to determine the best possible color match in the time allotted and thus must settle for a match that is often simply acceptable. In view of this, empirical approaches have been developed to aid the technician in quickly locating a limited region of the operating space of the proofing system in which a decent match can be achieved and to which the proofing system can be calibrated. One such empirical approach could involve first matching the C, M, Y and K solid area and halftone densities between the target image and the proof image to the extent realistically possible--though this may generally produce mismatches in overprint colors, e.g. the reds, greens and blues. Once these primary color matches are achieved, the resulting proof image is then visually examined to determine how certain overprint colors appear, e.g., whether gray tones are the same as those on the artwork or are too red. If the latter occurs, then the colorants are appropriately changed, possibly through successive iterative changes, to increase the cyan content or decrease the magenta and yellow content in the proof image. Alternatively, the technician could visually examine the reds in the proof image. If the reds appear too orange, the colorants could be appropriately changed to decrease the yellow content of the proof image or alternatively increase its magenta content. In that regard, it is widely known that an average human vision is acutely sensitive to flesh tones (which specifically contain red hues). Hence, even a subtle difference in coloration may be perceived as transforming an otherwise pleasant image of a human face into one that is quite unnatural and obnoxious. Through such approaches, even a skilled color technician may still need to generate upwards of 12-15 separate proof images in seriatim, typically requiring a full day of work, until he discerns the proper operating condition of the proofing system which is needed to achieve an acceptable color match between the target image and a proof image therefor and thereby calibrate the proofing system to the target imaging system in use.
Through a totally different approach, the technician could quantitatively measure reflection densities of selected portions of the image on both the target image and the proof image using, for example, a reflection densitometer, and then attempt to set the colorants in a manner that seeks to achieve the densities inherent in the target image. Unfortunately, this approach is constrained by the ability of the technician to locate corresponding relatively large uniformly colored areas on both the target image and the proof image at which the reflection densitometer can be reliably placed to take measurements. If both images contain significant detail, then suitable measurement areas may not exist and thereby preclude such densitometric measurements from being made. Moreover, as densitometers are designed specifically for measurement of amounts of colorants in an image and not for measurement of colors as perceived by humans, a densitometric match between the target and proof images will often be visually objectionable. For example, if the yellow colorant in the target image absorbed blue light over a wide band of wavelengths and the yellow colorant in the proof image absorbed blue light over a narrow band of wavelengths, a wide band densitometric match would lead to a proof image with a perceived excess of yellow colorant.
Apart from a reflection densitometer, one device that has recently become available for color measurement and matching is a spectrophotometer, such as the Model SPM 50 spectrophotometer manufactured by Gretag Corporation of Regensdorf, Switzerland. This device projects white light of a known spectral distribution onto an image, then separates the spectrum of reflected light from the image through a diffraction grating and thereafter measures the intensity of the reflected radiation at a number of different wavelengths. Through this device and its associated software, colorimetric spectral based measurements can be made of any reflection image. A commercially available software package (i.e. the "CMYK Conversion System" software) for use with this device and available from Gretag determines proper halftone dot size in the separations in order to achieve a desired coloration in the reflection image made therefrom. This software conceivably could be used to characterize (i.e. "model") a proofing system in use and then effectuate a color balance between a press sheet and a proof therefor. Specifically, a set of known test (reference or calibration) separations having numerous, complex and atypical patterns is provided with the device and software; the latter storing dot area settings for these separations. To characterize the proofing system, a proof image would be made from the reference separations. Thereafter, spectrophotometric measurements could be taken of this particular proof image. The resulting measurements, when processed, would yield a model that characterizes the color gamut producible through the proofing system. Thereafter, in order to generate a color match to a target image, the device could then be used to take spectrophotometric measurements of the target image. Given the characterization of the proofing system and the latter set of measurements, the software will determine appropriate values to use for solid area densities and corresponding halftone dot sizes for each primary colorant in the proofing system in order to generate a proof image that should match the target image for any particular color.
Inasmuch as the color gamut reproducible through a proofing system does not coincide with that appearing in the target image, the software used with this device, if used to match several colors simultaneously, is constrained, just as the technician is in manually performing a color match, to effectuate a compromise in matching the two gamuts between the target image and the proof image therefor. In achieving a color match, this software relies on the well-known CIELAB (L*a*b*) color coordinate system and color differences associated therewith. In computing a multiple color match, the software seeks to minimize an overall .DELTA.E value (i.e. a root sum squared length of multiple CIELAB color differences) between the two color gamuts and thus obtain an overall "colorimetric" match.
Even though the CIELAB system was designed to provide a nearly balanced measure of noticeable color differences, a relatively large colorimetric difference for some colors will lead to a relatively small .DELTA.E value; while this will not be true for other colors. Any system, such as the Gretag spectrophotometer, associated test separations and software, that seeks to minimize an overall colorimetric error between two images produced by systems with differing tone and color reproduction characteristics may well still produce minor color mismatches (as judged by their .DELTA.E values) for some colors that, in various image contexts, would be highly objectionable to a human observer.
In particular, it has been known for some time that human color perception, including mental judgment, exhibits differing sensitivities for different colors. Given this, human observers will be much more acutely aware of what would amount to minor color differences, such as differences in so-called "memory" colors (e.g., greens and flesh tones), in certain pictorial contexts than in others. Accordingly, a color difference that would simply be noticeable, if at all, in some contexts would be highly objectionable in others. For example, people are acutely aware of very small differences in flesh tones. A viewer will likely object to a human face that appears too blue or green, while merely noticing, if at all, and certainly not objecting to a tablecloth or blanket that exhibited the same variation. Thus, an effective color balance needs to account for the preferences inherent in human color perception. Specifically, if a target image is compared side-by-side to an accurate proof image thereof, a viewer should reach the conclusion that the proof image in effect has a good appearance, i.e. flesh tones appear as they should as well as do other colors given the context of the image thereon. In this instance, the relative coloration throughout the proof image is pleasing even though the specific hues in the proof image will not necessarily identically match those in the target image. Such a visually pleasing match between a proof image and a target image will hereinafter be referred to as an "appearance match".
Any system, such as the Gretag spectrophotometer and associated test separations and software, that attempts to provide a uniform "colorimetric" match across all colors using a metric based on context independent noticeability ignores the innate preferences inherent in human color perception. Consequently, the resulting proof image, based solely on such a colorimetric match to the target image, is likely to contain minor color differences, that depending upon the context of the particular image, can be highly objectionable to a viewer. Consequently, the proof image would not be a visually appealing representation of the target image. In these instances, an overall balance colorimetric based approach to color matching will clearly yield an unsatisfactory match that simply can not be used to calibrate a proofing system. When this occurs, a color technician would likely revert back to a manual approach to locate what he subjectively perceives to be an appearance match between the target image and the proof image--but will be forced to accept a rather high cost in time and material to do so.
As one can now appreciate, thus far the art has simply failed to provide a relatively fast systematic technique for objectively and automatically achieving a satisfactory appearance match between one color image, such as a press sheet, and another image, such as a proof therefor, for nearly all images, and particularly for use in calibrating one imaging system, such as a proofing system, to a target imaging system, such as a press.
Furthermore, proof images are typically printed, particularly through the proofing system described in the '459 Cowan et al patent, with so-called "run bars" that appear alongside the proof image. Each run bar provides a pre-defined sequence of color test patches composed of various combinations of primary colors, both as solids and halftones, as well as other diagnostic targets. These bars are intended to provide test areas for taking densitometric measurements of areas of uniform colors. For example, a typical run bar will contain, among others, separate solid and halftone color patches for the K, C, M and Y primary colors; separate solid color patches for the red (M and Y overprint), green (C and Y overprint), blue (C and M overprint) and three-color overprints (C, M and Y); and halftone color patches for a three-color overprint. Inasmuch as these bars exist alongside the proof and are formed by the proofing process using the same colorants as in the accompanying proof image itself, these bars identically represent the result of the proofing system tone and color reproduction characteristics in the proof image. Given the existence of the run bars in proof images, particularly those produced through the proofing system described in the '459 Cowan et al patent, it would be highly desirable to utilize these bars in some fashion to effectuate a color match between the proof image and a target image. Similar run bars are placed on target images, such as a press sheet. Alternatively, simple, easily generated and commonly used test objects such as step tablets comprising primary color, two color and three color overprint solids and tints could provide increased amounts of detail for specification of a tone reproduction curve shape, such as through the "Aim" dot gain capability of the DDCP system described in the '940 Spence application.
Therefore, a need currently exists in the art for a technique, specifically though not exclusively intended for inclusion in a proofing system, that can be used, particularly in conjunction with run bars or other simple test objects, to quickly, objectively and automatically provide an appearance match between an image produced by one imaging system, such as a proof image, and another image, such as a press sheet, produced from a common image source but by a different imaging system. While the colorations in such a match will rarely be identical due to differences in reproducible color gamut and color response between these two systems, the match attained through this technique should nevertheless result in, for example, a proof image that is, in substantially all instances, a visually accurate and appealing representation, i.e. an appearance match, of a target image. Moreover, by automatically providing an appearance match through objective criteria, such a technique should significantly reduce the trial-and-error effort and the degree of skill required of a user, as well as the associated time and cost, needed to achieve such a match and thereby calibrate a proofing system to a press or another proofing system.